The present invention relates to the structure of a vessel sealed with a double seal and, more particularly, to a sealed vessel whose air tightness property in each seal part can be easily checked and a method of testing the air tightness of the sealed vessel.
When an electronic apparatus is installed in the water such as on the sea bottom or in a river, the apparatus is usually housed in a pressure-proof sealed vessel. It is important to sometimes check the air tightness performance of the sealed vessel. In order to check the air tightness performance, an air tightness test using gaseous helium is usually carried out. A typical pressure-proof sealed vessel has a structure such that a lid is attached to an end face of a cylindrical vessel. The pressure-proof sealed vessel and the lid are usually sealed with a double seal. A through hole communicating the inside and outside of the pressure-proof sealed vessel is formed through the lid. A pin is inserted in the through hole and can be sealed by welding. In case of testing the vessel, a sealed vessel to be tested is placed in a pressure test vessel. A vacuum pump arranged outside of the pressure test vessel produces a near vacuum in the vessel via a pipe attached to the through hole. After that, the pipe is switched from the vacuum pump to a gas sensor and the gaseous helium is introduced into the pressure test vessel and is pressurized. When there is a leak in the seal part, the gas sensor senses the gaseous helium spread into the vessel. When the sealed vessel has the double seal, however, it is difficult to check the performance of each of the two seals. In the air tightness test, the pressure of the pressurized gaseous helium is applied to the outer seal and a pressure of about a normal air pressure is applied to the seal on the inside. It requires a long time air tightness test to verify that the gas is leaked from the inner seal part.
When a metal seal is employed as one of the seals, the gas does not permeate the seal part. In the conventional test, therefore, the air tightness performance of the other seal cannot be checked.